Water-resistant opaque papers for photographic purposes

ABSTRACT

The invention relates to opaque water-resistant papers and methods of making such papers suitable for use as base material for light-sensitive coatings for photographic purposes. According to the invention a sealing coat is applied to the paper base to prevent damage to the paper by application of the other coatings particularly aqueous or waterbound coatings.

United States Patent Inventor Gregor J. H. Kemme Ruckert-Strasse,Germany App]. No. 624,736 Filed Mar. 21, 1967 Patented Oct. 26, 1971Assignee Felix Schoeller, Jr.

Lustringen am Osnabruck, Germany Priority Mar. 3 1, 1966 Germany Sch38768 WATER-RESISTANT OPAQUE PAPERS FOR Primary Examiner-Ronald H. SmithAttorney-Hair, Freeman & Molinare ABSTRACT: The invention relates toopaque water-resistant papers and methods of making such papers suitablefor use as base material for light-sensitive coatings for photographicpurposes. According to the invention a sealing coat is applied to thepaper base to prevent damage to the paper by application of the othercoatings particularly aqueous or waterbound coatings.

WATER-RESISTAN T OPAQUE PAPERS FOR PHOTOGRAPHIC PURPOSES The inventionrelates to opaque water-resistant papers and a method of making opaquewater-resistant papers, suitable for use as base material forlight-sensitive coatings, that is to say as paper-based emulsionsupports, which, with the aid of a combination of coatings,-of which, onone side at least, one is light absorbent-are made waterproof on bothsides and opaque on one side and are thus suitable, in particular, foruse in self-developing cameras.

It is already a practice to employ water-protected papers inself-developing cameras for silver salt and color diffusion processes,with an opaque coating for daylight development applied to the backbefore the light-sensitive emulsions are applied. Such opaque backcoatings can be prepared by one or more applications of an aqueousplastics dispersion containing carbon black.

The coating of raw paper with aqueous dispersions of plastics hasdisadvantages, however, which come to the fore particularly in diffusiontransfer processes and affect the quality of photographs made with suchpapers. The action of water on the unprotected paper has an adverseefi'ect on the high-grade sheet and surface structure achieved by thepaper making machine, in the production of raw paper, by contactdryingon cylinders. Because of the partial swelling of the fibers associatedwith the irregularity of raw paper, because of the raising of thesurface fibers caused by water and because of the nonuniformity ofstress resulting from free drying in tunnel or hanging frames atnecessarily rather high temperatures, which, as is well known, detractfrom flatness, not only is the surface spoilt, but the structure of thesheet as a whole. These undesirable effects become visible after theapplication of light-sensitive emulsions, when these are exposed anddeveloped to a medium grey tone. They show up in photographs as a cloudydisturbance, termed mottle." It is too late for such mottling effectscaused by mishandling" of the raw paper to be entirely corrected bysmoothing the coated paper with the aid of calenders, for instance.

Many variations of method are known, whereby papers are extrusion coatedby means of wide-slit nozzles, and likewise the use of papers coated bythis method with thermoplastic substances for photographic purposes.This process offers a possibility of coating papers without adverselyaffecting their structure. Although it would thus be possible to applyopaque coatings to paper by extrusion, using mixtures of polyalkylenesand carbon black, with a high content of the latter, in a standardextrusion-coating installation, I have found, that it is not possible toproduce even faultless coatings with mixtures of polyalkylenes andcarbon black in which the content of the latter exceeds percent.However, if one employs the usual mixtures of polyalkylenes and carbonblack, containing 2 percent to 5 percent of the latter, a coatingapproximately 0.05 mm. to 0.06 mm. thick is needed to give the opacityrequired for daylight use, whereas a coating 0.02 mm. to 0.04 mm. thicksuffices if one uses carbon black lacquers or water bound colors. Even adifference in thickness of 0.02 mm. means, that the raw paper base mustbe worked correspondingly thinner, the natural irregularities in thepaper consequently being brought out more strongly. Experience showsthat this very factor results in enchanced mottling. Moreover, themechanical strength of polyalkylene films thicker than 0.03 mm. is sounfavorable as to make it questionable whether papers coated in this wayshould be used, for example, in self-developing cameras. It is indeedthe mechanical properties of coated papers which are so important incameras of the type in which negative and positive papers are insertedin roll form, because each individual photograph has to be torn offagainst a straight edge.

It is an object of the invention to provide a water-resistant opaquepaper for photographic purposes which has improved mottlingcharacteristics. l have now discovered that all the above adverseeffects on paper and picture quality can be avoided by the adoption of aprocess, for manufacturing the coated paper, in which a very thinsealing coat is applied by conventional means to the paper base, theopaque lacquers or suitable water bound colors being applied over thesealing coat.

The sealing coat may be applied by extrusion coating. All resins thatcan be extruded from the melt are suitable for this process. Forpreference, polyalkylenes with carbon-black fillers should be used. Thethickness of the coating, which will be determined by the structure ofthe paper as a whole and by the particular purposes for which it isrequired, should preferably be in the range 0.008 mm. to 0.02 mm.

However, the sealing coat may also be applied within the papermakingmachine by the use of water-soluble binders and/or aqueous dispersionsof plastics. A material feature in this case is that the addition ofwaterbound colors is carried out only when the drained paper has driedsufficiently to be entirely free from stresses. After a brief predryingof the coating, the coated paper is then dried throughout with the aidof heated cylinders. The sealing coat in this case should preferably bein the range 0.005 mm. to 0.015 mm.

Papers that have first been given a sealing coat on one side or bothsides by one or other of these methods, without adversely affecting thestructure of the sheet, can then be coated on one side for opacity andon the other for light reflection. This will result in an improvement inphotographic mottling, as compared to papers given a waterbound coatingin the usual way.

The photographic properties of papers coated in accordance with theinvention have been compared with those of papers manufactured in theconventional manner, a lightsensitive emulsion being applied to thecoated paper and exposed to light. After development or aftertransference of the image, comparison of the observable mottling ismade. Some typical results of this mottling test are summarized in tableI.

Specimen Nature of Coating Relative No. Moltling l Aqueous dispersion ofplastics 4-5 2 Cellulose lacquer in bulyl 23 acetate 3 Back:polyethylene lacquer 2 Front: lacquer 4 Back: polyethylene lacquer 2-3Front: polyethylene 5 Back: latex sealing coat 2-3 lacquer Front:polyethylene When a polyalkylene is used for the sealing coat, theadhesion of the outer coating can be achieved by conventional chemical,mechanical or electrical treatment of the polyalkylene surface. inparticular, electrical treatment of the surface by means of coronadischarge is a simple and reliable method, but it does not alwaysproduce adequate adhesion of the coatings in the case of nonpigmentedpolyalkylene. It is therefore proposed, in accordance with theinvention, that the sealing coat be pigmented with titanium dioxide orcarbon black, for example. Carbon black in particular is advantageous,in any technically usable portion, as regards the opacity required inthe end product. With pigmented sealing coats, the effect of electricalsurface treatment is that there is practically no limit to the choice ofbinders for the outer coating. One binding medium that is to bepreferred for lacquers is nitrocellulose, whereas preference should begiven to mixed polyacrylate polymers and/or gelatine as the bindingmedium for coatings made from an aqueous phase.

The thickness of the light-absorbent rear coating is approximately 0.015mm. to 0.03 mm. Carbon black should preferably be used by way oflight-absorbent pigment. Other light-absorbent substances may also beused, so long as they serve to provide or improve opacity. To testopacity, the following method may be employed. A highly sensitivephotographic paper (speed about 30/ DIN) is brought into contact withthe test piece in a printing frame. An exposure of 1 minute is thengiven, with an intensity of illumination of approximately 25,000 lux.When the photographic material thus exposed is developed, the nonopaqueportions show as black or grey dots on a white ground. Papers renderedopaque by the invention method show fewer then 10 pinhead-sized dots tothe square meter.

According to special requirements imposed on the base material forlight-sensitive emulsions, the opaque rear coating may have propertiesspecially called for in particular cases. Thus, for example, it may beantistatic, hot-sealing, matt and/or capable of being written upon.Moreover, it may have special sliding properties, as is particularlynecessary with certain self-developing cameras. These sliding propertiesare obtained either by the addition of the usual lubricating agents tothe opaque coating or, preferably, by the application of a separatelubricating coating characterized by the fact of its containing, inaddition to a suitable binder, finely dispersed graphite and/or finelydispersed tetrafluorethylene (co- )polymer.

Papers coated for opacity in accordance with the invention are used inthe self-developing camera as a base both for negative emulsions andalso for positive emulsions. On the other side, accordingly, they cannotonly be coated with ordinary white-pigmented lacquers or white-pigmentedpolyalkylene, but can also carry a coating of ordinary gelatine-sizedbarium sulfate.

As regards the process of image transfer by diffusion in selfdevelopingcameras, it is observed that papers rendered opaque and water-resistantin accordance with the invention, by extrusion-coating followed bylacquering or other coating, can be still further improved bycalendering after the coating processes. The calendering is carried outeither after the coating of the rear side or after completion of thebase and should preferably be done with the aid of highly polished steelrolls.

The following examples are given to provide a clearer understanding ofthe invention.

EXAMPLE I l. A very smooth white, untreated photographic paper (approx.80 g./sq.m.) is extrusion coated on one side with a mixture ofpolyethylene and carbon black (for example, 95:5 The thickness of thecoating is approx. 0.012 mm. The surface of the polyethylene iselectrically treated by corona discharge. 2. A standard lacquer made ofnitrocellulose, softener, carbon black and solvent is applied to thecoating of polyethylene and carbon black. The thickness of the driedcoat of lacquer is approximately 0.025 mm. The content of carbon blackis about percent in relation to the total solid.

3. A lubricating coating consisting, for example, of colloidal graphiteand ethyl cellulose in alcohol solution, is applied over the coating oflacquer.

4. The paper coated on one side as in (1H3) above is extrusion-coated onthe other side, in the conventional way, with a mixture of polyethyleneand titanium dioxide (85:15,) to a thickness of approximately 0.025 mm.The surface is rendered matt and is electrically treated. Thispolyethylene coating is coated with a normal adhesive forlight-sensitive emulsions.

EXAMPLE ll 1. An untreated photographic paper is painted on one side orboth sides, in the usual way, within the paper making machine with anormal waterbound color containing, for example, clay and/or titaniumdioxide (and perhaps also some carbon black) as the pigment, and reducedstarch and a commercial latex (Dow Latex 5l2-R, for example) as thebinder medium. The body content of the color and the addition of furthersecondary substances depend on the coating composition used and thecondition of the color. It is of advantage to adjust the body content to45-50 percent with the aid of a sizing press. The rate of applicationshould preferably be 8 to 10 g./sq.m. If necessary, to improve theadhesion of further coatings, a second coating device within thepapermaking machine may be used to apply a 0.5-3 percent solution of acolloidal preparation of silicic acid (such as Ludox).

2. Over the sealing coat appliedwithin the papermaking machine as in(l), a lacquer containing carbon black may be applied by conventionalmeans, to give the paper whatever properties are desired in eachparticular case (such as sealability, low friction, acceptance ofwriting and so forth). The carbon black content of the lacquer is about25 percent of the total solids. The coating is approximately 0.3 mm.thick.

3. The other side of the paper, already coated on one side for opacityin accordance with (l) and (2), is lacquered with an ordinarynitrocellulose lacquer containing titanium dioxide, on which an adhesivesubstratum for light-sensitive emulsions is added. Then the fully coatedbase material is calendered.

EXAMPLE Ill 1, A white photographic paper is given a sealing coat on oneside, as in example I or II.

2. Over this sealing coat, a waterbound color containing carbon black isapplied evenly with the aid of an airbrush, for example. By way ofhinder, the color contains a styrene/butadiene latex, which is filmforming at room temperature, and by way of pigment a 3:2 mixture oftitanium dioxide and carbon black. The ratio of pigment to binder is3:2.

3. Over this opaque combination coating, a pigment-free lubricatingcoating is applied, consisting of ethylcellulose in alcohol/toluol, forinstance.

4. The front side may be coated as in examples I or ll.

lclaim:

1. An improved photographic base paper consisting essentially of:

a. paper sheet having a front side and back side,

b. said front side having adhered thereto a nonaqueous polyalkylenesealing coat of thickness ranging from about 0,005 to 0.02 mm. andcontaining from 0 to about 15 percent titanium dioxide pigment,

c. said back side having adhered thereto a nonaqueous polyalkylenesealing coat of thickness ranging from about 0.005 to 0.02 mm. andcontaining from 0 to about 5 percent carbon black pigment,

d. said front side sealing coat having adhered thereto a water proofplastic overlayer selected from polyalkylene plastic and lacquercontaining titanium dioxide pigment, e. said backside sealing coathaving adhered thereto a water proof plastic overlayer selected frompolyalkylene plastic lacquer and styrene/butadiene latex containing apigment selected from carbon black and mixtures of carbon black andtitanium dioxide,

f. said overlayers having thickness of less than about 0.06

g. said base paper having a relative mottling of less than about 3, anopacity of less than about 10 inhead-sized dots per square meter, andexhibiting improved tear characteristics for use in self-developingcamera film.

2. A base paper as in claim 1 wherein said sealing coat polyalkylene ispolyethylene.

3. A base paper as in claim 2 which includes a photosensitive layeradhered to said front side overlayer.

4. A base paper as in claim 2 which includes a lubricating coatingapplied to said back side overlayer.

5. An improved photographic base paper consisting essentially of:

a. paper sheet having a front side and a back side,

b. said front side having adhered thereto a nonaqueous polyalkylenesealing coat of thickness ranging from about 0.005 to 0.02 mm. andcontaining from 0 to about percent titanium dioxide pigment,

c. said back side having adhered thereto a nonaqueous polyalkylenesealing coat of thickness ranging from about 0.005 to 0.02 mm. andcontaining from 0 to about 5 percent carbon black pigment,

d. said back side sealing coat having adhered thereto a water proofoverlayer selected from lacquer and styrene/butadiene latex containing apigment selected from carbon black, titanium dioxide, and mixturesthereof, said overlayer having a thickness of about 0.6 mm., and

e. said base paper having a relative mottling of less than about 3, anopacity of less than about I0 pinhead-sized dots per square meter, andexhibiting improved tear characteristics for use in self-developingcamera film.

a: s t r w

2. A base paper as in claim 1 wherein said sealing coat polyalkylene ispolyethylene.
 3. A base paper as in claim 2 which includes aphotosensitive layer adhered to said front side overlayer.
 4. A basepaper as in claim 2 which includes a lubricating coating applied to saidback side overlayer.
 5. An improved photographic base paper consistingessentially of: a. paper sheet having a front side and a back side, b.said front side having adhered thereto a nonaqueous polyalkylene sealingcoat of thickness ranging from about 0.005 to 0.02 mm. and containingfrom 0 to about 15 percent titanium dioxide pigment, c. said back sidehaving adhered thereto a nonaqueous polyalkylene sealing coat ofthickness ranging from about 0.005 to 0.02 mm. and containing from 0 toabout 5 percent carbon black pigment, d. said back side sealing coathaving adhered thereto a water proof overlayer selected from lacquer andstyrene/butadiene latex containing a pigment selected from carbon black,titanium dioxide, and mixtures thereof, said overlayer having athickness of about 0.6 mm., and e. said base paper having a relativemottling of less than about 3, an opacity of less than about 10pinhead-sized dots per square meter, and exhibiting improved tearcharacteristics for use in self-developing camera film.